Reducing noise in spectral data from polishing substrates

ABSTRACT

Among other things, a machine based method includes representing a plurality of spectra reflected from one or more substrates at a plurality of different positions on the one or more substrates in the form of a first matrix; decomposing, by one or more computers, the first matrix into products of at least two component matrixes of a first set of component matrixes; reducing dimensions of each of the at least two component matrixes to produce a second set of component matrixes containing the at least two matrixes with reduced dimensions; and generating, by the one or more computers, a second matrix by taking a product of the matrixes of the second set of component matrixes.

TECHNICAL FIELD

The present disclosure relates to reducing noise in spectral data frompolishing substrates, e.g., for controlling chemical mechanicalpolishing of the substrates.

BACKGROUND

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. A variety of fabrication processes requireplanarization of a layer on the substrate. For example, for certainapplications, e.g., polishing of a metal layer to form vias, plugs, andlines in the trenches of a patterned layer, an overlying layer isplanarized until the top surface of a patterned layer is exposed. Inother applications, e.g., planarization of a dielectric layer forphotolithography, an overlying layer is polished until a desiredthickness remains over the underlying layer.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier head. The exposed surface of thesubstrate is typically placed against a rotating polishing pad. Thecarrier head provides a controllable load on the substrate to push itagainst the polishing pad. A polishing liquid, such as slurry withabrasive particles, is typically supplied to the surface of thepolishing pad.

One problem in CMP is determining whether the polishing process iscomplete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness, or when a desired amount of material hasbeen removed. Variations in the initial thickness of the substratelayer, the slurry composition, the polishing pad condition, the relativespeed between the polishing pad and the substrate, and the load on thesubstrate can cause variations in the material removal rate. Thesevariations cause variations in the time needed to reach the polishingendpoint. Therefore, it may not be possible to determine the polishingendpoint merely as a function of polishing time.

In some systems, a substrate is optically measured in a stand-alonemetrology station. However, such systems often have limited throughput.In some systems, a substrate is optically monitored in-situ duringpolishing, e.g., through a window in the polishing pad. However,existing optical monitoring techniques may not satisfy increasingdemands of semiconductor device manufacturers.

SUMMARY

In one aspect, a machine based method comprises representing a pluralityof spectra reflected from one or more substrates at a plurality ofdifferent positions on the one or more substrates in the form of a firstmatrix; decomposing, by one or more computers, the first matrix intoproducts of at least two component matrixes of a first set of componentmatrixes; reducing dimensions of each of the at least two componentmatrixes to produce a second set of component matrixes containing the atleast two matrixes with reduced dimensions; and generating, by the oneor more computers, a second matrix by taking a product of the matrixesof the second set of component matrixes. Each spectrum is represented bya dataset arranged in a single row or a single column of the firstmatrix. The spectra is reflected when an outer layer of the one or moresubstrates has substantially the same thickness at the plurality ofpositions. The second matrix has the same dimensions as the firstmatrix. Each single row or each single column of the second matrixcomprises a modified dataset representing a modified spectrumcorresponding to the spectrum represented by a respective single row orsingle column of the first matrix.

In another aspect, a computer program product resides on a computerreadable medium, and the computer program product comprises instructionsfor causing a processor to: represent a plurality of spectra reflectedfrom one or more substrates at a plurality of different positions on theone or more substrates in the form of a first matrix; decompose, by oneor more computers, the first matrix into products of at least twocomponent matrixes of a first set of component matrixes; reducedimensions of each of the at least two component matrixes to produce asecond set of component matrixes containing the at least two matrixeswith reduced dimensions; and generate, by the one or more computers, asecond matrix by taking a product of the matrixes of the second set ofcomponent matrixes. Each spectrum is represented by a dataset arrangedin a single row or a single column of the first matrix. The spectra arereflected when an outer layer of the one or more substrates hassubstantially the same thickness at the plurality of positions. Thesecond matrix has the same dimensions as the first matrix. Each singlerow or each single column of the second matrix comprises a modifieddataset representing a modified spectrum corresponding to the spectrumrepresented by a respective single row or single column of the firstmatrix.

In another aspect, a computer system comprises a processor; a memory;and a storage device that stores a program for execution by theprocessor using the memory. The program comprises instructionsconfigured to cause the processor to: represent a plurality of spectrareflected from one or more substrates at a plurality of differentpositions on the one or more substrates in the form of a first matrix;decompose, by one or more computers, the first matrix into products ofat least two component matrixes of a first set of component matrixes;reduce dimensions of each of the at least two component matrixes toproduce a second set of component matrixes containing the at least twomatrixes with reduced dimensions; and generate, by the one or morecomputers, a second matrix by taking a product of the matrixes of thesecond set of component matrixes. Each spectrum is represented by adataset arranged in a single row or a single column of the first matrix.The spectra are reflected when an outer layer of the one or moresubstrates has substantially the same thickness at the plurality ofpositions. The second matrix has the same dimensions as the firstmatrix. Each single row or each single column of the second matrixcomprises a modified dataset representing a modified spectrumcorresponding to the spectrum represented by a respective single row orsingle column of the first matrix.

Certain implementations of the methods, the systems, and/or the computerprogram products may include one or more of the following features. Acharacterizing value is generated, by the one or more computers, basedon the second matrix. The characterizing value is associated with aproperty of the one or more substrates. The characterizing value is thethickness of the outermost layer on the one or more substrates. Spectrareflected from one or more substrates at locations where the outer layerof the one or more substrates has different thicknesses are grouped intodifferent groups. Each different group of spectra is to be representedby a first matrix. Decomposing the first matrix comprises applyingsingular value decomposition, CUR matrix approximation, and/or principalcomponent analysis. Reducing dimensions of each of the at least twocomponent matrixes comprises truncating one or more columns and/or oneor more rows of each of the at least two component matrixes. One of theat least two component matrixes is a diagonal matrix. The dimensions arereduced based on predetermined criteria. The predetermined criteriacomprise truncating the one or more columns and/or rows of the diagonalmatrix that correspond to one or more nonzero matrix elements having avalue smaller than a predetermined percentage of values of all nonzeromatrix elements of the diagonal matrix. The predetermined percentage is% or higher. The dimensions are reduced based on predetermined criteria.The predetermined criteria comprise the difference between the first andsecond matrixes being smaller than a predetermined value. Reducingdimensions of each of the at least two component matrixes comprisesreplacing all non-zero values of one or more columns or rows of matrixelements with zeros. One of the at least two component matrixes is adiagonal matrix and reducing dimensions of the diagonal matrix comprisesreplacing non-zero values of one or more diagonal matrix elements withzeros. The spectra are measured with an in-line monitoring system beforepolishing of the substrate. The spectra are measured with an in-situmonitoring system during polishing of the substrate. A polishingendpoint is determined for the substrate based on the characterizingvalue.

Certain implementations may include one or more of the followingadvantages. Noise in a spectrum can be reduced. Wafer thicknesses can bemeasured relatively precisely so that endpoints of polishing can bedetermined at a high precision. Wafer-to-wafer thickness non-uniformity(WIWNU and WTWNU) may be reduced, and reliability of the endpoint systemto detect a desired polishing endpoint may be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic cross-sectional view of an example of apolishing station.

FIG. 2 illustrates a schematic top view of a substrate having multiplezones.

FIG. 3 illustrates a schematic cross-sectional view of an example of anin-line monitoring station.

FIG. 4 illustrates a top view of a polishing pad and shows locationswhere in-situ measurements are taken on a substrate.

FIG. 5A illustrates a measured spectrum from the optical monitoringsystem.

FIG. 5B illustrates spectrum evolution as the thickness of a substratechanges during polishing.

FIG. 5C illustrates a schematic cross-sectional view of lightreflections and interferences in optical monitoring.

FIG. 5D illustrates a schematic plot of a part of a measured spectrumand a modified spectrum with reduced noise.

FIG. 5E illustrates another schematic cross-sectional view of lightreflections and interferences in optical monitoring.

FIG. 5F shows plots of an overall spectrum measured from the lightreflections and interferences of FIG. 5E, and of calculated spectra foreach individual light reflection and interference.

FIG. 5G shows a variety of factors that affect reflection spectra and inconnection with layers and features of a substrate.

FIG. 6 is a flow diagram of an example process that reduces noise inmeasured spectra.

FIG. 7 is a flow diagram of an example process that implements noisereduction in controlling a polishing operation.

DETAILED DESCRIPTION

One optical monitoring technique for controlling a polishing operationis to measure a spectrum of light reflected from a substrate, eitherin-situ during polishing or at an in-line metrology station, and fit afunction, e.g., an optical model, to the measured spectra. Anothertechnique is to compare the measured spectrum to a plurality ofreference spectra from a library, and identify a best-matching referencespectrum.

Either fitting of the optical model or identification of the bestmatching reference spectrum is used to generate a characterizing value,e.g., the thickness of the outermost layer. For the fitting, thethickness can be treated as an input parameter of the optical model, andthe fitting process generates a value for the thickness. For finding amatch, the thickness value associated with the reference spectrum can beidentified.

Chemical mechanical polishing can be used to planarize the substrateuntil a predetermined thickness of the first layer is removed, apredetermined thickness of the first layer remains, or until the secondlayer is exposed.

Generally, the measured spectrum contains noise data that may affect theprecision of the characterizing value or the thickness measurement. Themethods and systems of this disclosure reduce the noise in the spectrumto improve the precision of thickness measurement, although precision ofmeasurement of other properties can also be improved. The preciselydetermined thickness can be used to determine the endpoint of thechemical mechanical polishing and improve the uniformity of the waferthickness.

FIG. 1 illustrates an example of a polishing apparatus 100. Thepolishing apparatus 100 includes a rotatable disk-shaped platen 120 onwhich a polishing pad 110 is situated. The platen is operable to rotateabout an axis 125. For example, a motor 121 can turn a drive shaft 124to rotate the platen 120. The polishing pad 110 can be a two-layerpolishing pad with an outer polishing layer 112 and a softer backinglayer 114.

The polishing apparatus 100 can include a port 130 to dispense polishingliquid 132, such as a slurry, onto the polishing pad 110. The polishingapparatus can also include a polishing pad conditioner to abrade thepolishing pad 110 to maintain the polishing pad 110 in a consistentabrasive state.

The polishing apparatus 100 includes one or more carrier heads 140. Eachcarrier head 140 is operable to hold a substrate 10, such as a wafer,against the polishing pad 110. Each carrier head 140 can haveindependent control of the polishing parameters, for example pressure,associated with each respective substrate. Each carrier head includes aretaining ring 142 to hold the substrate 10 in position on the polishingpad 110.

Each carrier head 140 is suspended from a support structure 150, e.g., acarousel or a track, and is connected by a drive shaft 152 to a carrierhead rotation motor 154 so that the carrier head can rotate about anaxis 155. Optionally each carrier head 140 can oscillate laterally,e.g., on sliders on the carousel 150; by rotational oscillation of thecarousel itself, or by motion of a carriage 108 that supports thecarrier head 140 along the track.

In operation, the platen is rotated about its central axis 125, and eachcarrier head is rotated about its central axis 155 and translatedlaterally across the top surface of the polishing pad.

While only one carrier head 140 is shown, more carrier heads can beprovided to hold additional substrates so that the surface area ofpolishing pad 110 may be used efficiently. Thus, the number of carrierhead assemblies adapted to hold substrates for a simultaneous polishingprocess can be based, at least in part, on the surface area of thepolishing pad 110.

In some implementations, the polishing apparatus includes an in-situoptical monitoring system 160, e.g., a spectrographic monitoring system,which can be used to measure a spectrum of reflected light from asubstrate undergoing polishing. An optical access through the polishingpad is provided by including an aperture (i.e., a hole that runs throughthe pad) or a solid window 118.

Referring to FIG. 2, if the window 108 is installed in the platen, dueto the rotation of the platen (shown by arrow 204), as the window 108travels below a carrier head, the optical monitoring system makingspectra measurements at a sampling frequency will cause the spectrameasurements to be taken at locations 201 in an arc that traverses thesubstrate 10.

In some implementation, illustrated in FIG. 3, the polishing apparatusincludes an in-sequence optical monitoring system 160 having a probe 180positioned between two polishing stations or between a polishing stationand a transfer station. The probe 180 of the in-sequence monitoringsystem 160 can be supported on a platform 106, and can be positioned onthe path of the carrier head.

The probe 180 can include a mechanism to adjust its vertical heightrelative to the top surface of the platform 106. In someimplementations, the probe 180 is supported on an actuator system 182that is configured to move the probe 180 laterally in a plane parallelto the plane of the track. The actuator system 182 can be an XY actuatorsystem that includes two independent linear actuators to move probe 180independently along two orthogonal axes. In some implementations, thereis no actuator system 182, and the probe 180 remains stationary(relative to the platform 106) while the carrier head 140 moves to causethe spot measured by the probe 180 to traverse a path on the substrate.

Referring to FIG. 4, the probe 180 can traverse a path 184 over thesubstrate while the monitoring system take a sequence of spectrameasurements, so that a plurality of spectra are measured at differentpositions on the substrate. By proper selection of the path and the rateof spectra measurement, the measurements can be made at a substantiallyuniform density over the wafer. Alternatively, more measurements can bemade near the edge of the substrate.

In the specific implementation shown in FIG. 4, the carrier head 126 canrotate while the carriage 108 causes the center of the substrate to moveoutwardly from the probe 180, which causes the spot 184 measured by theprobe 180 to traverse a spiral path 184 on the substrate 10. However,other combinations of motion can cause the probe to traverse otherpaths, e.g., a series of concentric circles or a series of arcuatesegments passing through the center of the substrate 10. Moreover, ifthe monitoring station includes an XY actuator system, the measurementspot 184 can traverse a path with a plurality of evenly spaced parallelline segments. This permits the optical metrology system 160 to takemeasurements that are spaced in a rectangular pattern over thesubstrate.

Returning to FIGS. 1 and 3, in either the in-situ or in-sequenceembodiments, the optical monitoring system 160 can include a lightsource 162, a light detector 164, and circuitry 166 for sending andreceiving signals between a remote controller 190, e.g., a computer, andthe light source 162 and light detector 164. One or more optical fiberscan be used to transmit the light from the light source 162 to theoptical access in the polishing pad, and to transmit light reflectedfrom the substrate 10 to the detector 164. For example, a bifurcatedoptical fiber 170 can be used to transmit the light from the lightsource 162 to the substrate 10 and back to the detector 164. Thebifurcated optical fiber an include a trunk 172 positioned in proximityto the optical access, and two branches 174 and 176 connected to thelight source 162 and detector 164, respectively. The probe 180 caninclude the trunk end of the bifurcated optical fiber.

The light source 162 can be operable to emit white light. In oneimplementation, the white light emitted includes light havingwavelengths of 200-800 nanometers. In some implementations, the lightsource 162 generates unpolarized light. In some implementations, apolarization filter 178 (illustrated in FIG. 3, although it can be usedin the in-situ system of FIG. 1) can be positioned between the lightsource 162 and the substrate 10. A suitable light source is a xenon lampor a xenon mercury lamp.

The light detector 164 can be a spectrometer. A spectrometer is anoptical instrument for measuring intensity of light over a portion ofthe electromagnetic spectrum. A suitable spectrometer is a gratingspectrometer. Typical output for a spectrometer is the intensity of thelight as a function of wavelength (or frequency). In some examples, foreach spectrum, the spectrometer outputs the intensities of the light at200-500, e.g., 301, different wavelengths. FIG. 5A shows an example ofspectra reflected from the substrate by applying a single flash of lightfrom a light source to the substrate. In particular, spectrum 602 ismeasured from light reflected from a product substrate. Spectrum 604 ismeasured from light reflected from a base silicon substrate (which is awafer that has only a silicon layer). Spectrum 606 is from lightreceived by the probe 180 when there is no substrate situated over theprobe 180. Under this condition, referred to in the presentspecification as a dark condition, the received light is typicallyambient light.

The computing device can process the above described signal to determinean endpoint of a polishing step. Without being limited to any particulartheory, the spectra of light reflected from the substrate 10 evolve aspolishing progresses. FIG. 5B provides an example of the evolution aspolishing of a film of interest progresses. The different lines ofspectrum represent different times in the polishing. As can be seen,properties of the spectrum of the reflected light changes as a thicknessof the film changes, and particular spectrums are exhibited byparticular thicknesses of the film. The computing device can executelogic that determines, based on one or more of the spectra, when anendpoint has been reached. The one or more spectra on which an endpointdetermination is based can include a target spectrum, a referencespectrum, or both.

As noted above, the light source 162 and light detector 164 can beconnected to a computing device, e.g., the controller 190, operable tocontrol their operation and receive their signals. The computing devicecan include a microprocessor situated near the polishing apparatus,e.g., a programmable computer. In operation, the controller 190 canreceive, for example, a signal that carries information describing aspectrum of the light received by the light detector for a particularflash of the light source or time frame of the detector.

Referring to FIGS. 5A and 5C, the spectra 602 contains information abouta thickness 612 of an outer layer 650 of the substrate 10. Duringpolishing, the outer layer directly contacts the polishing pad 110 andits outer surface 614 is polished. The substrate 10 may have one or moreadditional layers 610 at the back of the outer layer 650. When a flashof white light 620 is applied to the substrate 10, part of the whitelight 620 reflects (622) at the outer surface 614 of the outer layer650. Another part of the white light 620 penetrates the outer surface614 and is at least partially reflected (624) by an inner surface 616 ofthe outer layer 650. Part of the light 620 may further penetrate theinner surface 624 and be at least partially reflected by the one or morelayers 610 or a surface 618 at the back of the substrate 10.

Without wishing to be bound by any particular theory, it is believedthat the spectra data contains information about the thickness(es) ofone or more layers of the substrate 10. The values of the spectra, i.e.,the light intensities may also be affected by the optical properties ofthe layers, such as index of refractions of the substrate materials, theabsorption coefficients of the substrate materials, and others. It isalso believed that the light intensity in the spectrum 604 also containsinformation representing interferences between reflections, such asthose between the reflections 624, 626 or the reflections 622, 626. Thelight intensity data may also contain other noise data that originatefrom sources, such as variation in structural dimensions of the device(e.g., critical dimensions, sidewall angle, etc.), pad window variations(e.g., changing absorption in the shorter wavelengths), other processinfluences (e.g., slurry pooling).

As an example, referring to FIGS. 5E and 5F, an example substrate 10includes a stack of layers 902, 904, 906, 908, 910, 912, 914, 916, withone or more layers containing one or more structural features 918, 920,922, etc. When a flash of light is applied to an outer surface 924 ofthe substrate 10, the light penetrates one or more layers 902, 904 andis reflected from multiple surfaces 924, 926, 928. The measured spectra1000 shown in FIG. 5F can be a weighted combination of respectivespectra 1002, 1004, 1006 associated with the different reflections fromthe different surfaces. As shown in FIG. 5G, the factors that affect themeasured spectra may include thickness variations of the layers in thesubstrate, the substrate material (which is Cu in this example),diffractions due to Cu grating by patterning, lightscattering/interference at sidewalls, etc.

Reducing the noise of the spectrum 604 can allow the thickness 612 ofthe outer layer 650 to be determined at a higher precision than thespectrum 604 that contains the noise data. As an example, FIG. 5D showsa spectrum 604 recording the output of the spectrometer. After the noiseis reduced, the modified spectrum 642 shows a relatively smooth curveand retains the largest component of the spectrum 604 representing thethickness of the outer layer 650.

Alternative to the plots shown in FIGS. 5A, 5B, and 5D, a spectrummeasured at one location of the substrate 10 can also be represented bya matrix:

B=(i₁, i₂, . . . , i_(n)), where i_(j) represents the light intensity atthe jth wavelength of a total of n wavelengths. In an example, n can be200-500, or 301.

Referring again to FIG. 2, when multiple spectra are measured atmultiple locations 201 of the substrate during one rotation of thepolishing pad, each spectrum can be similarly presented by a matrix B.In some situations, the thicknesses measured at the multiple locations201 within the same rotation are substantially the same. For example,the thickness variation among the different locations measured withinthe same rotation would be within 500 Angstroms. Suppose that mlocations are measured, the m matrixes B can be combined to form thefollowing matrix:

${A = \begin{pmatrix}i_{11} & i_{12} & \ldots & i_{1\; n} \\i_{21} & i_{22} & \cdots & i_{2\; n} \\\vdots & \vdots & \ldots & \vdots \\i_{m\; 1} & i_{m\; 2} & \ldots & i_{mn}\end{pmatrix}},$where i_(jk) represents the light intensity at the kth wavelength of thejth location. Each row of the matrix A represents a spectrum forcalculating a thickness of the substrate at one location, while allthicknesses represented by the different spectra are substantially thesame. Alternatively, data of a spectrum can also be arranged in a singlecolumn.

The noise data contained in the spectra represented by the matrix A canbe reduced using singular value decomposition (SVD), CUR matrixapproximation, or principal component analysis, each followed bydimension reductions. These techniques can identify similar componentsin a dataset, e.g., thickness in this example, while filtering out noisecomponents in the dataset, e.g., due to variations in the thickness ofan underlying layer. The techniques can be used in the in-situ systemsor the in-line systems. The techniques can also be used with otherdatasets, e.g., data stored in a data library or database.

Singular Value Decomposition

The decomposition can be mathematically expressed as follows:A=UΣV ^(T),where U and V are orthonormal matrixes, U^(T)U=I, and V^(T)V=I, having mby p dimensions and n by p dimensions, respectively, and Σ is diagonaland has p by p dimensions. Without wishing to be bound by theory, it isbelieved that each column of the U matrix and each row of the V^(T)matrix represent a concept, such as different degrees, or strengths, ofinterferences between different reflection light in the thicknessmeasurement. It is also believed that each element of the U matrixrepresents location-to-concept similarities, the Σ matrix representsstrength of each concept, and each element of the V matrix representswavelength-to-concept similarities.

The U matrix can be written as:

${U = ( {u_{1},u_{2},\ldots\mspace{14mu},u_{p}} )},{{{where}\mspace{14mu} u_{i}} = \begin{pmatrix}u_{1\; i} \\u_{2\; i} \\\vdots \\u_{mi}\end{pmatrix}},{i.e.},{U = \begin{bmatrix}u_{11} & \ldots & u_{1\; p} \\\vdots & \ddots & \vdots \\u_{m\; 1} & \ldots & u_{mp}\end{bmatrix}}$the V matrix can be written as:

${V = ( {v_{1},v_{2},\ldots\mspace{14mu},v_{p}} )},{{{where}\mspace{14mu} v_{i}} = \begin{pmatrix}v_{1\; i} \\v_{2\; i} \\\vdots \\v_{ni}\end{pmatrix}},{i.e.},{V^{T} = \begin{bmatrix}v_{11} & \ldots & v_{n\; 1} \\\vdots & \ddots & \vdots \\v_{1\; p} & \ldots & v_{np}\end{bmatrix}},{and}$the Σ matrix can be written as:

${\Sigma = ( {\sigma_{1},\sigma_{2},{\ldots\mspace{14mu}\sigma_{p}}} )},{{{where}\mspace{14mu}\sigma_{i}} = {\begin{pmatrix}0 \\\vdots \\\sigma_{ii} \\\; \\\vdots \\0\end{pmatrix}.}}$For every matrix A, the decomposition is unique Also, the Σ matrix isarranged such that σ₁₁≧σ₂₂≧ . . . ≧σ_(pp). Accordingly, the matrix A canbe written as:A=σ ₁ u ₁ v ₁ ^(T)+σ₂ u ₂ v ₂ ^(T)+ . . . +σ_(p) u _(p) v _(p) ^(T)

Following the decomposition, to reduce the noise data in the A matrix,the dimensions of the U, Σ, and V^(T) matrixes are effectively reduced.In some implementations, the values of σ_(ii) decreases as i increases,such that:Σ_(i>t)|σ_(ii)|² <r*Σ _(i)|σ_(ii)|²,where t is an integer between 1 and p, and r is a predeterminedpercentage, e.g., 60%, 70%, 80%, 90%, or higher. In suchimplementations, those matrix element(s) σ_(ii) that are part of thesmall values Σ_(i>t)|σ_(ii)|² limit the values or significance of thecorresponding term Σ_(i>t)σ_(i)u_(i)v_(i) ^(T) in the matrix A, suchthat these terms only correspond to a small fraction, e.g., less than40%, less than 30%, less than 20%, less than 10%, or less, of the matrixA. Such a small fraction can correspond to the undesirable noise in thespectra. Accordingly, these terms Σ_(i>t)σ_(i)u_(i)v_(i) ^(T) can bediscarded.

In some implementations, for simplicity, a predetermined number ofnon-zero diagonal elements of the Σ matrix is eliminated or selected toremain in the matrix. For example, two diagonal elements having thesmallest values are eliminated from the Σ matrix, or two diagonalelements having the largest values are chosen to remain in the matrixwhile all other diagonal elements are eliminated. Those rows and columnsof the matrix that correspond to the eliminated diagonal elements can beeliminated to reduce the dimensions of the matrix. Alternatively, thevalues of the diagonal elements to be eliminated can be replaced withzero.

Effectively, the U matrix is reduced to have m by t dimensions, the Vmatrix is reduced to have n by t dimensions, and the Σ matrix is reducedto have t by t dimensions. This reduction in dimension could beaccomplished by actually removing rows and column from the matrices Uand V, respectively, or by simply setting the appropriate diagonalvalues in the Σ matrix to zero. The matrix A is reconstructed by thematrixes with reduced dimensions to be approximately:A≈σ ₁ u ₁ v ₁ ^(T)+σ₂ u ₂ v ₂ ^(T)+ . . . +σ_(t) u _(t) v _(t) ^(T),where the large components of the matrix A remains in the reconstructedmatrix A. Each row of the reconstructed matrix A represents a modifiedspectrum that corresponds to a measured spectrum in the original matrixA. Relative to the measured spectrum, the modified spectrum contains areduced amount of noise and can be used to calculate the thickness ofthe outermost layer of the substrate. Such calculated thicknesses aremore precise than thicknesses calculated based on the original matrix A,because of the noise reduction.

Alternative or in addition to the criteria for dimension reductiondiscussed above, the difference between the original and thereconstructed matrixes can also be used. For example, the difference,∥original A−reconstructed A∥² is chosen to be smaller than apredetermined value so that the dimension reduction does not reduce thedata in addition to the noise data, e.g., the data needed forcalculating the thickness.

CUR Matrix Approximation

In some implementations, alternative to the singular valuedecomposition, CUR matrix approximation can be used to decompose theoriginal A matrix. The method can be selected based the properties ofthe A matrix, for example, how sparse the A matrix is, etc. Sometimesthe CUR matrix approximation is chosen when the A matrix is relativelysparse, although the singular value decomposition can also be used.

Principal Component Analysis

Another alternative decomposition method is principal component analysis(PCA). The PCA performs an orthogonal linear transformation thattransforms the data in the A matrix (m×n dimensions) to a new coordinatesystem such that the greatest variance by any projection of the datacomes to lie on the first coordinate (called the first principalcomponent), the second greatest variance on the second coordinate, andso on. Mathematically, the transformation is defined by a set ofp-dimensional vectors of weights w_(k)=(w_(k1), w_(k2), . . . , w_(kp))that map each m-dimensional row vector A_(i) of the matrix A to a newvector of principal component scores t_(i)=(t_(k1), t_(k2), . . . ,t_(ip)), where t_(ki) is:t _(ki) =A _(i) ·w _(k).Each vector w_(k) is constrained to be a unit vector. As a result, theindividual variables of t_(i) inherits the maximum possible variancefrom the matrix A. The decomposition of the matrix A can be written as:T=AW,where W is a n-by-p matrix whose columns are the eigenvectors of A^(T)A.To reduce the noise data in the matrix A, the dimensions of the T and Wmatrixes can be reduced. In particular, instead of p principalcomponents, a total of L principal components, with L being an integerbetween 0 and p, can be kept such that the dimensions of the T matrix isreduced to m×L, and the dimensions of the W matrix is reduced to n×L.Similar to the singular value decomposition, a matrix A can bereconstructed based on the T and W matrixes with reduced dimensions. Theconstructed matrix A can have all the major components that representthe thickness of the substrate, while a large portion of the noise datais removed as compared to the original matrix A.

The PCA transformation can be mathematically associated with thepreviously discussed singular value decomposition, and the T matrix canbe expressed as:T=UΣ.Details of the PCA are also discussed in Functional Data Analysis byJames Ramsay and B. W. Silverman (Springer; 2nd edition (Jul. 1, 2005)),and Principal Component Analysis by I. T. Jolliffe (Springer; 2ndedition (Oct. 2, 2002)), the entire content of which is incorporatedherein by reference.

The noise reduction processes discussed above can be used in processingmeasured spectra before the data is used in calculating substratethicknesses in an in-situ system or an in-line system. The noisereduction processes can be performed in real time during the polishingor be applied to data from a database or a data library that hascollected the data.

An example process 800 is shown in FIG. 6. After the spectra, e.g.,light intensity data which typically is set for one thicknessmeasurement, or other data, are collected, or are made available, e.g.,through links to a database or data library, datasets that containinformation for a similar, e.g., the same, thickness are binned (802)together. The datasets binned together can be data for one wafer, e.g.,collected for a given rotation of the platen, or data for multiplewafers, e.g., from a data library, that is associated with the similaror the same thickness. The binned data can be stored in the form of amatrix, like the matrix A shown above. Optionally, the binned data isfurther clustered (804) based on different types of spectra for a givenrotation and/or a given thickness value. One technique for theclustering is k-means clustering, the details of which are discussed inU.S. Ser. No. 14/063,740, filed on the same day as the presentapplication, the entire content of which is incorporated herein byreference. The clustering may further refine the binned datasets tofacilitate precise thickness calculations. The clustered data for agiven thickness value can also be stored in the form of a matrix, suchas the matrix A above. The matrix is then decomposed (806), usingsingular value decomposition, principal component analysis, or CURmatrix approximation, into component matrixes (e.g., U, Σ, and Tmatrixes in SVD, or T and W matrixes in PCA). The dimensions of thecomponent matrixes are then reduced (808). The reduction can be doneautomatically based on predetermined criteria. For example, in theexample of SVD, the predetermined criteria can be whether a percentageof the values of the matrix elements, starting with the one having thehighest index, in the Σ matrix is lower than a threshold percentage. Analgorithm can be implemented such that when the percentage of the valuesof those elements is lower than the threshold percentage, which can bepredetermined, columns and rows corresponding to those matrix elementsare truncated from the matrix. The dimensions of U and V matrixes arereduced according to the reduction in the Σ matrix. Other criteria fordimension reduction can also be used. A user may be enabled to enter oradjust the predetermined criteria, e.g., through a user interface. Insome implementations, the dimension reduction can be manually performedby a user. The datasets, or the spectra, are reconstructed (810) usingthe matrixes with the reduced dimensions. The reconstructed datasets orspectra contain a reduced amount of noise as compared to the initialdatasets or spectra.

Referring again to FIG. 1, for each reconstructed dataset or spectrum,the controller 190 can calculate a characterizing value. Thecharacterizing value is typically the thickness of the substrate 10,e.g., the thickness 612 of FIG. 5C, but can be a related characteristicsuch as thickness removed. In addition, although not discussed in detailabove, the characterizing value can be a physical property other thanthickness, e.g., metal line resistance. In addition, the characterizingvalue can be a more generic representation of the progress of thesubstrate through the polishing process, e.g., an index valuerepresenting the time or number of platen rotations at which thespectrum would be expected to be observed in a polishing process thatfollows a predetermined progress. Details of techniques for calculatingthe characterizing value are discussed in U.S. Pat. No. 7,764,377 andU.S. Ser. No. 13/777,672, the entire content of which is incorporatedhere by reference.

FIG. 7 shows an example process 700 of implementing the noise reductiondiscussed above in controlling polishing of a product substrate. Aplurality of spectra reflected from the product substrate are collected(702) at a plurality of different positions during one rotation of theplaten. The spectra could be measured using an in-sequence opticalmonitoring system or an in-situ optical monitoring system. Noisereduction is performed (703) on the collected spectra. Optionally,before applying the noise reduction, the spectra may be sorted using aclustering algorithm such as the k-means clustering algorithm. The noisereduction can be performed on each cluster of spectra. An example of thenoise reduction process is the process 800 of FIG. 6. Characterizingvalues, e.g., thicknesses, can be extracted (704) from the spectra withreduced noise. The characterizing values could be generated byidentifying a matching reference spectrum from a library of referencespectra, or by fitting an optical model to the collected spectrum. Thecharacterizing values are then used (706) in controlling polishing ofthe product substrate. For example, when the characterizing values arethicknesses, the calculated thicknesses can be compared with apredetermined value to determine the endpoint of the polishing process.In another example, thicknesses at different moments of the polishingprocess can be determined and a polishing rate can be derived from thethicknesses. The thicknesses can also be used to determine the criteriafor dimension reduction in the noise reduction process. For example, thedetermined thicknesses can be compared with directly measuredthicknesses to determine the effectiveness of the noise reductionprocess and/or adjust the criteria.

As used in the instant specification, the term substrate can include,for example, a product substrate (e.g., which includes multiple memoryor processor dies), a test substrate, a bare substrate, and a gatingsubstrate. The substrate can be at various stages of integrated circuitfabrication, e.g., the substrate can be a bare wafer, or it can includeone or more deposited and/or patterned layers. The term substrate caninclude circular disks and rectangular sheets.

The above described polishing apparatus and methods can be applied in avariety of polishing systems. Either the polishing pad, or the carrierheads, or both can move to provide relative motion between the polishingsurface and the substrate. For example, the platen may orbit rather thanrotate. The polishing pad can be a circular (or some other shape) padsecured to the platen. Some aspects of the endpoint detection system maybe applicable to linear polishing systems, e.g., where the polishing padis a continuous or a reel-to-reel belt that moves linearly. Thepolishing layer can be a standard (for example, polyurethane with orwithout fillers) polishing material, a soft material, or afixed-abrasive material. Terms of relative positioning are used; itshould be understood that the polishing surface and substrate can beheld in a vertical orientation or some other orientation.

Although the description above has focused on control of a chemicalmechanical polishing system, the in-sequence metrology station can beapplicable to other types of substrate processing systems, e.g., etchingor deposition systems.

Embodiments, such as noise reduction or controlling substrate polishing,of the subject matter and the functional operations described in thisspecification can be implemented in digital electronic circuitry, intangibly-embodied computer software or firmware, in computer hardware,including the structures disclosed in this specification and theirstructural equivalents, or in combinations of one or more of them.Embodiments of the subject matter described in this specification can beimplemented as one or more computer programs, i.e., one or more modulesof computer program instructions encoded on a tangible non transitorystorage medium for execution by, or to control the operation of, dataprocessing apparatus. Alternatively or in addition, the programinstructions can be encoded on an artificially generated propagatedsignal, e.g., a machine-generated electrical, optical, orelectromagnetic signal, that is generated to encode information fortransmission to suitable receiver apparatus for execution by a dataprocessing apparatus. The computer storage medium can be amachine-readable storage device, a machine-readable storage substrate, arandom or serial access memory device, or a combination of one or moreof them.

The term “data processing apparatus” refers to data processing hardwareand encompasses all kinds of apparatus, devices, and machines forprocessing data, including by way of example a programmable digitalprocessor, a digital computer, or multiple digital processors orcomputers. The apparatus can also be or further include special purposelogic circuitry, e.g., an FPGA (field programmable gate array) or anASIC (application specific integrated circuit). The apparatus canoptionally include, in addition to hardware, code that creates anexecution environment for computer programs, e.g., code that constitutesprocessor firmware, a protocol stack, a database management system, anoperating system, or a combination of one or more of them.

A computer program, which may also be referred to or described as aprogram, software, a software application, a module, a software module,a script, or code, can be written in any form of programming language,including compiled or interpreted languages, or declarative orprocedural languages, and it can be deployed in any form, including as astand alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A computer program may, butneed not, correspond to a file in a file system. A program can be storedin a portion of a file that holds other programs or data, e.g., one ormore scripts stored in a markup language document, in a single filededicated to the program in question, or in multiple coordinated files,e.g., files that store one or more modules, sub programs, or portions ofcode. A computer program can be deployed to be executed on one computeror on multiple computers that are located at one site or distributedacross multiple sites and interconnected by a data communicationnetwork.

The processes and logic flows described in this specification can beperformed by one or more programmable computers executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit). For a system of one or morecomputers to be “configured to” perform particular operations or actionsmeans that the system has installed on it software, firmware, hardware,or a combination of them that in operation cause the system to performthe operations or actions. For one or more computer programs to beconfigured to perform particular operations or actions means that theone or more programs include instructions that, when executed by dataprocessing apparatus, cause the apparatus to perform the operations oractions.

Computers suitable for the execution of a computer program include, byway of example, can be based on general or special purposemicroprocessors or both, or any other kind of central processing unit.Generally, a central processing unit will receive instructions and datafrom a read only memory or a random access memory or both. The essentialelements of a computer are a central processing unit for performing orexecuting instructions and one or more memory devices for storinginstructions and data. Generally, a computer will also include, or beoperatively coupled to receive data from or transfer data to, or both,one or more mass storage devices for storing data, e.g., magnetic,magneto optical disks, or optical disks. However, a computer need nothave such devices. Moreover, a computer can be embedded in anotherdevice, e.g., a mobile telephone, a personal digital assistant (PDA), amobile audio or video player, a game console, a Global PositioningSystem (GPS) receiver, or a portable storage device, e.g., a universalserial bus (USB) flash drive, to name just a few.

Computer readable media suitable for storing computer programinstructions and data include all forms of non volatile memory, mediaand memory devices, including by way of example semiconductor memorydevices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks,e.g., internal hard disks or removable disks; magneto optical disks; andCD ROM and DVD-ROM disks. The processor and the memory can besupplemented by, or incorporated in, special purpose logic circuitry.

Control of the various systems and processes described in thisspecification, or portions of them, can be implemented in a computerprogram product that includes instructions that are stored on one ormore non-transitory machine-readable storage media, and that areexecutable on one or more processing devices. The systems described inthis specification, or portions of them, can be implemented as anapparatus, method, or electronic system that may include one or moreprocessing devices and memory to store executable instructions toperform the operations described in this specification.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinvention or on the scope of what may be claimed, but rather asdescriptions of features that may be specific to particular embodimentsof particular inventions. Certain features that are described in thisspecification in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various system modulesand components in the embodiments described above should not beunderstood as requiring such separation in all embodiments, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

Particular embodiments of the subject matter have been described. Otherembodiments are within the scope of the following claims. For example,the actions recited in the claims can be performed in a different orderand still achieve desirable results. As one example, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In some cases, multitasking and parallel processing may beadvantageous.

What is claimed is:
 1. A machine-based method comprising: representing aplurality of spectra reflected from one or more substrates at aplurality of different positions on the one or more substrates in theform of a first matrix, each spectrum being represented by a datasetarranged in a single row or a single column of the first matrix, and thespectra being reflected when an outer layer of the one or moresubstrates has substantially the same thickness at the plurality ofpositions; decomposing, by one or more computers, the first matrix intoproducts of at least two component matrixes of a first set of componentmatrixes; reducing dimensions of each of the at least two componentmatrixes to produce a second set of component matrixes containing the atleast two matrixes with reduced dimensions; and generating, by the oneor more computers, a second matrix by taking a product of the matrixesof the second set of component matrixes, the second matrix having thesame dimensions as the first matrix, and each single row or each singlecolumn of the second matrix comprising a modified dataset representing amodified spectrum corresponding to the spectrum represented by arespective single row or single column of the first matrix.
 2. Themethod of claim 1, comprising generating, by the one or more computers,a characterizing value based on the second matrix, the characterizingvalue being associated with a property of the one or more substrates. 3.The method of claim 2, wherein the characterizing value is the thicknessof the outer layer on the one or more substrates.
 4. The method of claim2, comprising determining a polishing endpoint for the substrate basedon the characterizing value.
 5. The method of claim 1, comprisinggrouping spectra reflected from one or more substrates at locationswhere the outer layer of the one or more substrates has differentthicknesses into different groups, each different group of spectra to berepresented by a first matrix.
 6. The method of claim 1, whereindecomposing the first matrix comprises applying singular valuedecomposition, applying CUR matrix approximation, or applying principalcomponent analysis.
 7. The method of claim 1, wherein reducingdimensions of each of the at least two component matrixes comprises i)truncating one or more columns and/or one or more rows of each of the atleast two component matrixes, or ii) replacing all non-zero values ofone or more columns or rows of matrix elements with zeros.
 8. The methodof claim 1, wherein the plurality of spectra are measured with anin-line monitoring system before polishing of the substrate.
 9. Themethod of claim 1, wherein the plurality of spectra are measured with anin-situ monitoring system during polishing of the substrate.
 10. Acomputer program product residing on a computer readable medium, thecomputer program product comprising instructions for causing a processorto: represent a plurality of spectra reflected from one or moresubstrates at a plurality of different positions on the one or moresubstrates in the form of a first matrix, each spectrum beingrepresented by a dataset arranged in a single row or a single column ofthe first matrix, and the spectra being reflected when an outer layer ofthe one or more substrates has substantially the same thickness at theplurality of positions; decompose, by one or more computers, the firstmatrix into products of at least two component matrixes of a first setof component matrixes; reduce dimensions of each of the at least twocomponent matrixes to produce a second set of component matrixescontaining the at least two matrixes with reduced dimensions; andgenerate, by the one or more computers, a second matrix by taking aproduct of the matrixes of the second set of component matrixes, thesecond matrix having the same dimensions as the first matrix, and eachsingle row or each single column of the second matrix comprising amodified dataset representing a modified spectrum corresponding to thespectrum represented by a respective single row or single column of thefirst matrix.
 11. The computer program product of claim 10, comprisinginstructions to generate, by the one or more computers, a characterizingvalue based on the second matrix, the characterizing value beingassociated with a property of the one or more substrates.
 12. Thecomputer program product of claim 10, wherein the instructions to reducedimensions of each of the at least two component matrixes compriseinstructions to i) truncate one or more columns and/or one or more rowsof each of the at least two component matrixes, or ii) replace allnon-zero values of one or more columns or rows of matrix elements withzeros.
 13. The computer program product of claim 12, wherein one of theat least two component matrixes is a diagonal matrix and theinstructions to reduce dimensions comprise instructions to replacenon-zero values of one or more diagonal matrix elements with zeros. 14.The computer program product of claim 12, wherein one of the at leasttwo component matrixes is a diagonal matrix and the instructions totruncate comprise instruction to truncate the one or more columns and/orrows of the diagonal matrix that correspond to one or more nonzeromatrix elements having a value smaller than a predetermined percentageof values of all nonzero matrix elements of the diagonal matrix.
 15. Thecomputer program product of claim 14, wherein the predeterminedpercentage is 80% or higher.
 16. The computer program product of claim10, wherein the instructions to reduce dimensions comprise instructionsto reduce dimensions based on a predetermined criterion, and thepredetermined criterion comprises a difference between the first andsecond matrixes being smaller than a predetermined value.
 17. Apolishing system, comprising: a support to hold a polishing pad; acarrier head to hold a substrate comprising at least two regions havingdifferent structural features in contact with the polishing pad; anin-situ optical monitoring system to measure a plurality of measuredspectra reflected from a substrate at a plurality of different positionson the substrate, the spectra being reflected when an outer layer of thesubstrate has substantially the same thickness at the plurality ofpositions; and a controller configured to receive the plurality ofmeasured spectra from the in-situ optical monitoring system, representthe plurality of spectra in the form of a first matrix, each spectrumbeing represented by a dataset arranged in a single row or a singlecolumn of the first matrix, decompose the first matrix into products ofat least two component matrixes of a first set of component matrixes,reduce dimensions of each of the at least two component matrixes toproduce a second set of component matrixes containing the at least twomatrixes with reduced dimensions, generate, by the one or morecomputers, a second matrix by taking a product of the matrixes of thesecond set of component matrixes, the second matrix having the samedimensions as the first matrix, and each single row or each singlecolumn of the second matrix comprising a modified dataset representing amodified spectrum corresponding to the spectrum represented by arespective single row or single column of the first matrix, generate acharacterizing value based on the second matrix, the characterizingvalue being associated with a property of the one or more substrates,and determine a polishing endpoint for the substrate based on thecharacterizing value.
 18. The polishing system of claim 17, wherein thecharacterizing value is the thickness of the outer layer on thesubstrates.
 19. The polishing system of claim 17, wherein the controlleris configured to reduce dimensions of each of the at least two componentmatrixes by i) truncating one or more columns and/or one or more rows ofeach of the at least two component matrixes, or ii) replacing allnon-zero values of one or more columns or rows of matrix elements withzeros.